This invention relates to an air cycle cooling system and more particularly but not exclusively to an air cycle cooling system for use in cooling a heat load such as that produced in an aircraft by the operation of avionics equipment and/or radar equipment or other electrically operated apparatus, and/or a heat load produced in an aircrew enclosure, i.e. for air conditioning.
Air cycle air conditioning is widely used in aircraft to provide conditioned air whilst taking advantage of a supply of pressurised air bled from an aircraft engine, known as bleed air. However in modem high performance aircraft for example, engines are being designed to be increasingly fuel efficient and as a result less pressurised bleed air is available for ancillary systems such as for an air cycle cooling system. Accordingly in such an air cycle cooling system there is a requirement to reduce dependence on engine bleed air as a source of pressurised air.
According to a first aspect of the invention we provide an air cycle cooling system for cooling a first heat load and a second heat load, the system including a compressor to pressurise air in the system, and an expansion apparatus for allowing the compressed air to expand and cool for use in cooling the first and second heat loads, and wherein at least a proportion of the air utilised for cooling the first heat load is recycled to the compressor and at least a proportion of the air utilised for cooling the second heat load is utilised as a coolant in a primary heat exchanger to cool the air from the first heat load prior to the air from the first heat load being expanded in the expansion apparatus.
Utilising an air cycle cooling system in accordance with the invention for cooling first and second heat loads in an aircraft, dependence upon engine bleed air as a source of pressurised air may be reduced whilst a simple yet effective and energy efficient system may be provided.
The compressor may include first and second compressor stages, each compressor stage including a compressor wheel, and the primary heat exchanger may be positioned between the first and second compressor stages so that the air from the first heat load which is cooled by the air from the second heat load is pre-compressed by the first compressor stage.
In one arrangement the compressor wheels are driven by a common electric motor and thus the compressor wheels may be carried on a common drive shaft. In another arrangement, individual electric motors may be provided for driving the compressor wheels of the first and second compressor stages.
The system may include a fan for assisting cooling of the pressurised air prior to feeding the pressurised air to the expansion apparatus.
Further to improve the efficiency of the system of the invention, energy recovered from the pressurised air supply in the expansion apparatus may be utilised to provide power to drive at least one of the electric motor or motors for driving the compressor, and the fan. For example the recovered energy may be utilised to drive a power generator for driving at least one of the compressor and the fan.
The cooling system may in one embodiment be an at least partially closed loop system in which at least some of the air which has been utilised for cooling the heat load is subsequently recycled to the compressor for re-pressurisation. In such an arrangement an auxiliary pressurised air supply would be required to make up air lost from the system.
Such auxiliary pressurised air may be pre-cooled prior to mixing with re-pressurised air from the compressor, for example in a pre-cooling heat exchanger, in which the auxiliary air may be pre-cooled by exchanging heat with a coolant being air. For example, where the system is provided in an aircraft, the coolant may be ram air, that is air which is induced to flow into the heat exchanger, by the movement of the aircraft through the air, and/or air at ambient temperature.
In an aircraft, such an auxiliary pressurised air supply may be engine bleed air. However, only a relatively small amount of engine bleed air would be required for such make-up purposes and accordingly the requirement to reduce dependence on engine bleed air as a source of pressurised air, may still be satisfied.
Whether the auxiliary pressurised air supply is engine bleed air or another source of pressurised air, preferably the auxiliary air and the re-pressurised recycled air from the compressor are mixed and cooled prior to expansion and cooling in the expansion apparatus, for example by cooling the auxiliary and recycled air in a secondary heat exchanger. Heat in the auxiliary and recycled air may be exchanged with a coolant in the secondary heat exchanger, such coolant including air for example ram air and/or air at ambient temperature. The coolant air may be induced to flow through the secondary heat exchanger by the fan where operative, or where the system is in an aircraft, by the movement of the aircraft through the air.
The invention has been developed particularly but not exclusively for providing cooling of first and second heat loads in an aircraft. Thus the heat load may include a first heat load produced by for example, an electrically operated apparatus such as avionics equipment and/or radar equipment in an aircraft and a second heat load produced in an aircrew or passenger enclosure.
In each case of an at least partially closed or open loop system, where the heat load includes a heat load produced in an aircrew or passenger enclosure, a hot air supply may be made available to the aircrew or passenger enclosure when heating of the enclosure is required, for example hot compressed air from the compressor. Moreover in each case of an at least partially closed or open loop system, a water separator may be provided to separate water from the air expanded and cooled in the expansion apparatus. Such water may be utilised for assisting cooling of the pressurised air prior to expansion and cooling in the expansion apparatus. For example such water may be sprayed into the air coolant in the secondary heat exchanger where provided, to improve the efficiency of cooling in the secondary heat exchanger.
According to a second aspect of the invention we provide an air cycle cooling system for cooling a heat load, the system including a compressor to pressurise an air supply, and an expansion apparatus for allowing the compressed air to expand and cool for use in cooling the heat load, and wherein the compressor is electrically driven by a motor, and the system further includes at least one of a fan for assisting cooling of the pressurised air prior to feeding the pressurised air to the expansion apparatus, and a power generator for generating power for assisting driving the compressor or a fan, the fan and/or the power generator being driven by energy recovered from the pressurised air supply in the expansion apparatus, the system being a partially closed system in which at least some of the air which has been utilised for cooling the heat load is recycled to the compressor for re-pressurisation.
According to a third aspect of the invention we provide an air cycle cooling system for cooling a heat load, the system including a compressor to pressurise an air supply, and an expansion apparatus for allowing the compressed air to expand and cool for use in cooling the heat load, and wherein the compressor is electrically driven by a motor, and the system further includes at least one of a fan for assisting cooling of the pressurised air prior to feeding the pressurised air to the expansion apparatus, and a power generator for generating power for assisting driving the compressor or a fan, the fan and/or the power generator being driven by energy recovered from the pressurised air supply in the expansion apparatus, the system being an open loop system in which at least a proportion of the air utilised for cooling the heat load is discharged from the system subsequently to being utilised for cooling air to be fed to the expansion apparatus.
The system of the second or third aspect of the invention may have any of the features of the system of the first aspect of the invention.
According to a fourth aspect of the invention we provide a method of operating an air cycle cooling system for cooling a heat load, the system including a compressor to pressurise an air supply, and an expansion apparatus for allowing the compressed air to expand and cool for use in cooling the heat load, and wherein the method includes driving the compressor electrically by a motor, and recovering energy from the pressurised air supply in the expansion apparatus and using the recovered energy for one of driving a fan for assisting cooling of the pressurised air prior to feeding the pressurised air to the expansion apparatus and driving a power generator for generating power for assisting driving the compressor or a fan.
The method of the fourth aspect of the invention may utilise an air cycle cooling system having any of the features of the first, second or third aspects of the invention.